1. Field of the Invention
The present invention relates to an image forming apparatus that controls positioning and magnification of a main scanning image.
2. Description of the Related Art
In image forming apparatuses employing a light beam scanning device, a light beam is modulated according to image data, subjected to an equal angular velocity deflection in a main scanning direction by rotation of a deflecting unit such as a polygon mirror, and further subjected to, for example, a correcting process of the equal angular velocity deflection to an equal velocity deflection using an fθ lens, and then the light beam is scanned on an image carrier (hereinafter, “photoconductor”).
However, image magnification differs among different apparatuses due to fluctuation in characteristics of parts such as lenses constituting respective light beam scanning devices. Characteristics in a main scanning direction vary even in a single apparatus, and therefore, magnification errors occur. Particularly, when a plastic lens is used, a shape and a refraction index of the lens vary due to changes in the environmental temperature or changes in the temperature in the apparatus. Therefore, a scanning position on the photoconductor varies, and a magnification error in the main scanning direction occurs, such that a high quality image cannot be obtained. In an apparatus in which a plurality of color images are formed using a plurality of laser beams and lenses, color registration deviation occurs due to respective magnification errors, such that an image with high quality cannot be obtained. Accordingly, it is necessary to match image magnifications of respective colors as much as possible by reducing full width magnification errors and partial magnification errors of the respective colors.
One approach is to correct such an image magnification error in the main scanning direction, so that fluctuation among different machines and different colors can be reduced by variably controlling a pixel clock frequency.
However, since actual magnification characteristics in the main scanning direction are uneven along the main scanning direction and magnification errors differ at different portions along the main scanning direction, a mismatch among image magnifications or a positional deviation among images occurs partially. In color image forming apparatuses that form images of plural colors, there is a possibility that a color registration deviation occurs partially among respective portions in the main scanning direction.
In view of these circumstances, Japanese Patent Application Laid-Open Nos. 2004-4510, 2004-85777, H9-174917, and 2003-185953, and Japanese Patent No. 3231610 disclose a technique for correcting an image magnification error in the main scanning direction and a main scanning image position deviation generated by characteristics of parts such as a lens or a polygon mirror constituting a light beam scanning device, or generated due to fluctuations of the characteristics, in an image forming apparatus that forms images by conducting scanning of a light beams.
In Japanese Patent Application Laid-Open No. 2004-4510, partial magnification correction is performed with high accuracy by changing a phase of a pixel clock output for each period obtained by dividing a constant scanning period of a laser beam at intervals shorter than a pixel clock period.
In Japanese Patent Application Laid-Open No. 2004-85777, partial magnification corrections of respective colors except for a reference color in a color image forming apparatus are performed by designating magnification for each section in the main scanning direction and changing a clock speed for each predetermined number of counts, thereby preventing color registration deviation.
In Japanese Patent Application Laid-Open No. H9-174917, data for correcting an expansion and contraction distortion measured in advance for respective facets of a rotary polygon mirror are stored for three periods of a reference period, a short period, and a long period of a pixel clock, all dots in the main scanning direction are divided into a plurality of sections, and respective facet distortions are corrected, while data items for the three periods are being applied, so that a color image with high quality can be obtained.
In Japanese Patent No. 3231610, data for correcting an expansion and contraction distortion measured in advance for respective facets of a rotary polygon mirror are stored for three periods of a reference period, a short period, and a long period of a pixel clock, all dots in the main scanning direction are divided into a plurality of sections, and respective facet distortions are corrected, while data for the three periods are applied, so that a color image with high quality can be obtained. All dots in the main scanning direction are divided into a plurality of sections, and deviation amounts between an actual boundary position and a reference boundary position in respective sections are obtained in advance before an optical system is assembled to a main unit, so that deviation is corrected for each section based on the deviation amounts. A position deviation detecting pattern is formed on a transfer belt, so that deviation is corrected for each section based on a deviation amount from a predetermined interval.
In Japanese Patent Application Laid-Open No. 2003-185953, sensors that detect a light beam are provided on a writing start side and a writing termination side outside an effective writing region, and each dot position of image data within the effective writing region is corrected to an arbitrary position based on a fluctuation amount of a scanning time between the sensors, thereby suppressing occurrence of an color registration deviation. The effective writing region is divided into a plurality of image data regions, and a correction value is set for each image data region, thereby reducing data amount and downsizing a controller.
Image magnification and an image position can be corrected at a corresponding position by changing a phase or a frequency of a pixel clock (making a width of an image clock longer or shorter). However, it is necessary to detect an image magnification deviation amount and an image position deviation amount required for the correction using a sensor, a charge coupled device (CCD), or the like, as described in Japanese Patent Application Laid-Open Nos. 2004-4510, 2004-85777, H9-174917, and 2003-185953.
For example, to correct image magnification at a point at which a deviation amount is detected, it is necessary to correct pixel clocks positioned on a scanning start side with respect to the point. Even if pixel clocks positioned downstream from the point are corrected, an image magnification error occurs corresponding to the correction, such that errors further occur in image magnification downstream from the point.
As described in Japanese Patent Application Laid-Open Nos. 2004-4510, 2004-85777, H9-174917, and 2003-185953 and Japanese Patent No. 3231610, it is necessary to obtain a deviation amount before providing the optical system in the apparatus main unit to perform correction based on the deviation amount, or it is necessary to form a pattern for detecting a position deviation on a transfer belt after providing the optical system in the apparatus main unit to conduct correction based on a deviation amount from the predetermined distance. However, even if both of these corrections are conducted, position deviation of an image might occur on recording paper due to a problem in a conveying state of the paper (e.g. an angle at which the paper is conveyed).
By mounting as many sensors for detecting a position deviation detecting pattern in a main unit of an apparatus as possible, a magnification error in the main scanning direction can be detected and corrected with higher precision. However, an increase in the number of sensors increases the cost of the main unit, and therefore, in reality, not many sensors can be mounted in the main unit.